Many pourable food products, such as fruit juice, UHT milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of this type of package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing laminated strip packaging material.
The packaging material has a multilayer structure substantially comprising a base layer for stiffness and strength, which may comprise a layer of fibrous material, e.g. paper, or mineral-filled polypropylene material; and a number of layers of heat-seal plastic material, e.g. polyethylene films, covering both sides of the base layer.
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of gas- and light-barrier material, e.g. aluminium foil or ethyl vinyl alcohol (EVOH) film, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
According to a first known technique, packages of this sort are produced on fully automatic packaging units, on which a continuous tube is formed from the web-fed packaging material; the web of packaging material is sterilized on the packaging unit, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, once sterilization is completed, is removed from the surfaces of the packaging material, e.g. evaporated by heating; and the web of packaging material so sterilized is maintained in a closed, sterile environment, and is folded and sealed longitudinally to form a vertical tube.
The tube is fed continuously in a first vertical direction, is filled with the sterilized or sterile-processed food product, and is gripped at equally spaced cross sections by two pairs of jaws. More specifically, the two pairs of jaws act cyclically and successively on the tube, and heat seal the packaging material of the tube to form a continuous strip of pillow packs connected to one another by respective transverse sealing bands, i.e. extending in a second direction perpendicular to said first direction.
The pillow packs are separated by cutting the relative transverse sealing bands, and are then fed to a final folding station where they are folded mechanically into the finished parallelepiped shape.
According to a second alternative technique, the packaging material may be cut into blanks. In this case, blanks are firstly erected to form sleeves which are sealed at their bottom ends. Afterwards, the sleeves are filled with the pourable product through their open top ends, and top ends are sealed, so as to complete the formation of packages. Once formed, packages are sterilized.
In both cases, the packaging material in which the layer of barrier material comprises a sheet of electrically conductive material, e.g. aluminium, is normally heat sealed by a so-called induction heat-sealing process, in which, a loss current is induced in, and locally heats, the aluminium sheet, thus melting the heat-seal plastic material locally.
More specifically, in induction heat sealing, the sealing device substantially comprises an inductor powered by a high-frequency current generator and substantially comprising one or more inductor bars made of electrically conductive material, and which interact with the packaging material to induce a loss current in it and heat it to the necessary sealing temperature.
In case that packages are formed starting from a tube of packaging material, the sealing device is fitted to a first jaw. The other jaw, known as the counter-jaw, comprises a counter-sealing element fitted with pressure pads made of elastomeric material, and which cooperate with the sealing device to heat seal the tube along a relative transverse sealing band. In detail, the sealing device locally melts the two layers of heat-seal plastic material gripped between the jaws.
Furthermore, the counter-jaw houses in sliding manner a cutting element. In particular, the cutting element may slide towards and away from the sealing device of the sealing jaw along a third direction orthogonal to first and second direction.
In case that packages are formed starting from relative blanks of packaging material, the sealing device is fitted to a jaw of a packaging unit.
In both cases, known sealing devices substantially comprise a supporting body which defines two front seats for housing respective inductor bars; and an insert made of magnetic flux-concentrating material—in particular, a composite material comprising ferrite—and housed inside the supporting body, close to the inductor bars.
More precisely, inductor bars comprise relative active surfaces which are arranged on outer surface of sealing device, and cooperate with the packaging material during the formation of packages.
Active surfaces also comprise respective projections which are intended to cooperate with packaging material and increase the pressure thereon, so causing the mixing of the melted plastic material of the packaging material in the sealing area.
Though performing excellently on the whole, sealing device of the type described leave room for improvement.
In greater detail, a first need is felt within the industry to avoid that short circuits be produced between the active surfaces.
As a matter of fact, short circuits between the active surfaces may prevent the induced loss current from flowing within the packaging material so impairing the final quality of heat-sealing. Furthermore, short-circuits may damage the inductor bars and may result in an interruption of the heat-sealing process.
In particular, short circuits may be produced when the packaging material is provided with a plurality of pierceable portions about which a frame of an opening device will be fitted.
Each pierceable portion of the package may be defined by a so-called “prelaminated” hole, i.e. a hole formed in the base layer only and covered by the other lamination layers, including the layer of gas-barrier material.
In case it forms the layer of gas-barrier material, the aluminium layer could get in contact with the active surfaces and create an electrical bridge therebetween.
Short circuits between active surfaces may be also caused by residues of highly saline food products, for example dog food containing a saline gel, which lay between the active surfaces of the inductor bars.
Such kind of short circuit may especially occur when packages are formed from blanks. This is mainly due to the fact that the filling of sleeves with food product and the sealing of top ends of sleeves generates splashes of food product onto and between the active surfaces of the inductor bars.
Furthermore, in the case of packages formed from blanks, sealing device seals top ends of two sleeves at the same time and need, therefore, to be powered with a very high voltage, so increasing the risk of short-circuits.
A second need is felt within the industry to avoid that inductor bars and especially their projections are corroded.
As a matter of fact, the corrosion of projections and/or of part of the inductors adjacent thereto could reduce the pressure exerted by active surfaces onto packaging material and mixing of the melted plastic material of packaging material, so reducing the overall quality of the sealing.